How to gradually reflect the texture and leave the material?
You can make a mask with black and white (image or a gradient texture) and then use [Object] texture coordinate with an Empty selected to determine the physical location of that texture. You can then animate the location of that mask by either animating the mapping node location, or by animating the location of the empty itself. This also works for scale and rotation.

Use this mask then on a mix shader between two shaders, one with blank and one with the materials for the covers.